1. Field of the Art
This invention relates to a collimator lens for collimating light rays from a light source to produce a parallel light beam, particularly suitable for use in an optical pickup system for recording and reproducing information on optical discs or similar optical recording media.
2. Prior Art
Generally, optical pickups in use for recording and reproducing data on an optical disc or a similar recording medium are constituted by a laser light source in the form of a laser diode or the like, an objective lens for converging a laser light beam from the light source into a minute beam spot toward an optical disc surface, and a photo-detector for detecting reflected light coming from the optical disc. Further, since light rays from the light source are divergent light rays, the optical pickup is provided with a collimator lens to collimate the light rays, along with a beam splitter and a holographic optical element for splitting light paths of an output light beam of the light source and reflected signal light from an optical disc.
In this connection, in consideration of problems of follow-up actions in auto focusing, it is necessary for an optical system of the optical pickup to be able to suppress aberrations to a minimum. Especially, in a case where a laser diode of a short wavelength of approximately 400 nm is employed for the purpose of enhancing the density of information, it is necessary to correct to a sufficient degree not only spherical aberrations but also axial chromatic aberrations because the wavelength of the laser diode dependent on temperature to give rise to the problem of so-call wavelength leaping.
In this regard, described in Japanese Laid-Open Patent Application H5-127078 is an optical pickup with an optical system which is arranged to correct axial chromatic aberrations of an objective lens. In the case of this Japanese Laid-Open Patent Application H5-127078, an objective lens is constituted by a cemented lens which is composed of two spherical glass lenses, i.e., a biconvex lens and a meniscus lens. An aspheric plastic lens is laminated on one convex surface of the biconvex lens of the objective lens, which faces toward a recording disc surface.
However, in this case, the objective lens in the form of a cemented glass lens is required to be an extremely bright lens with an NA as large as 0.53, which however involves difficulties in lens designing and results in a very costly optical system. It is possible to correct aspheric aberrations by laminating a plastic film of an aspheric lens on a surface of the cemented glass lens, but this method of correcting spherical aberrations by making an objective lens surface aspheric requires an aspheric lens surface of a complicate shape with a greater degree of ups and downs. Generally, a plastic lens is laminated on a glass lens by the use of a stamping die, but this method is not always feasible because difficulties are usually encountered in forming thick aspheric lenses by stamping.
On the other hand, described in Japanese Laid-Open Patent Application H6-331888 is a collimator lens which is imparted with a function of correcting spherical aberrations, i.e., an aspheric collimator lens having an ellipsoid of revolution of a positive refraction power on a collimated light discharging side and a spherical surface on the opposite side. In this case, the collimator lens can correct spherical aberrations and sine condition satisfactorily in an NA range of 0.15 to 0.3.
As mentioned above, the prior art collimator lens is arranged to correct both spherical aberrations and sine condition to a satisfactory degree in a low NA range of NA=0.15 to 0.3. There arises no problem in particular, for example, at a wavelength of 660 nm, a wavelength for DVD (Digital Versatile Disc). However, in case an optical pickup employs as a light source a laser diode of a short wavelength, for example, a laser diode of 408 nm in a blue wavelength range, it becomes necessary to solve a new problem of axial aberrations from the standpoint of realizing higher densification of information.
The optical arrangements described in above-mentioned Japanese Laid-Open Patent Application H6-331888 is not for an optical pickup with a light source of a short wavelength because it is difficult to correct axial chromatic aberrations to a practically acceptable degree. Besides, although there is no description with regard to lens material in Japanese Laid-Open Patent Application H6-331888, fabrication of an aspheric glass lens is difficult and very costly.